Retrotransposition events have been well documented in yeast and Drosophila for many years. In these organisms much is known about the details of this mysterious biological phenomenon. In man, until recently we could only surmise that such events occurred, although the presence of highly repeated human elements with sequence homology to retrotransposons of other species strongly suggested their existence. In 1987 our lab found two instances of de novo retrotransposition of L1 sequences causing de novo hemophilia A in man. In this application we first propose to finish tracking the functional precursor of one of these elements and to describe thoroughly its subfamily and the subfamilies' evolution. We will then isolate the functional precursor of the second L1 element which retrotransposed and compare the structure of these two elements. These studies are critical to our knowledge of the biology and propagation of L1 elements. Second, we will use short oligonucleotide probes specific for each subfamily to search for new insertions in de novo cases of autosomal dominant and X-linked dominant disorders. We should be able to clone the gene for one of these disorders through use of the sequence flanking the inserted L1 element. Third, we will study the expression of the functional element found in the first aim in vivo and in vitro. We will discover whether element found in the first aim in vivo and in vitro. We will discover whether this sequence is transcribed after transfection in a "natural state" without an overexpressing promoter, whether the open-reading frames are translated in cells, and whether the second open-reading frame can code for a natural reverse transcriptase. All of these studies should provide fundamental answers to questions as to how certain L1 elements act as retrotransposons. The second aim should lead us to a novel scientific use of these rare events, the cloning of a hitherto unknown disease gene.